KB2742599 fails to update

Security Update for Microsoft .NET Framework 3.5.1 on Windows 7 and Windows Server 2008 R2 SP1 for x64-based Systems (KB2742599)

Download size: 8.0 MB

You may need to restart your computer for this update to take effect.

Update type: Important

A security issue has been identified that could allow an unauthenticated remote attacker to compromise your system and gain control over it. You can help protect your system by installing this update from Microsoft. After you install this update, you may have to restart your system.

More information:
http://go.microsoft.com/fwlink/?LinkID=261909

Help and Support:
http://support.microsoft.com

Error Code: 800B0100

I've tried several times but this just won't update. Thanks

We also need at least the latest two CBSPersist files - there's no significant data in the CBS.log file.
The CheckSUR log is clear, at least.